In the food service business, there is a rush at meals imposing at least two "peak demand" periods on cooking facilities leaving business during the rest of the day on a "stand-by" basis.
Originally there evolved a concept called the "flywheel effect" pertaining to the sufficiency of heat stored in "working" fat during "stand-by" which was required to meet the peak demands for cooking. The reason for this storage of heat was to permit the cooking of food which had a greater immediate heat requirement than that which the equipment was currently able to supply and to prevent the development of excessive temperature gradients within the fat under the sudden infusion of large amounts of heat which would quickly deteriorate the fat and produce greasy fried foods. Under this concept, fryers were designed with large reserves of fat so that much "non-working" fat was tied up in the fat-frying process.
In 1952, the state of the art changed when a new fryer design evolved which dispensed with the "flywheel effect" effect and minimized the amount of "non-working" fat held in reserve in the fat-containing vessel. It was found that the fat heat-up rate could be increased from 8.86.degree. /minute F., as provided for in the American Standards requirements, to as much as 56.degree. /minute F. without damage to the fat therein if there was proper minimization of "non-working" fat. Faster heat-up, of course, made the fryer more capable of meeting the short order demands of the trade. A smaller amount of non-working fat tied up in reserve also made the fryer more economical in the use of fat.
The reduced need for reserve fat in the fryer also extended the life of the working fat. By working all fat to a more uniform temperature and exposing less fat to the atmosphere, such fryers actually resulted in less contamination during the fat-frying duty cycle.
As fat at cooking temperature deteriorates, the level of free fatty acid therein increases, and at some point the fat takes on an unpleasant flavor. A 41/2 to 5% free fatty acid composition is generally considered as the top limit of acceptability. With a high turnover of make-up fat wherein, say, a quantity of fat equal to the total working and non-working fat in the fat-containing vessel is added as make-up fat over a period of 24 hours, the residual fat in the fat-frying duty cycle can establish an equilibrium composition well below this acceptable level of free fatty acid content and as long as the fat taken out of the vessel by absorption in the food is replenished, the residual fat need not be changed for an unlimited time. Such an equilibrium condition has been possible in the fat-containing vessels of the past having large fat reserves and operating under the so-called "flywheel" concept with frying of potato chips, which are sliced from 3 to 4 inch diameter potatoes and no thinner than 16 to 22 slices/inch, or very small cut French fries which are less than 1/8 to 3/16 inch square and up to 3 to 4 inches long each having high surface-to-volume ratio and because the high fat-absorptive characteristic of food products in these small sections makes a high turnover of make-up fat necessary.
Othe causes for the shortening of fat life in deep fat fryers come from food particle contamination of the fat. Commercial fryers must use conventional tools for cleaning fat fryers and removing crumbs and food particles which have settled from the frying portion to the bottom of the heating portion of the fat-containing vessel. These contaminants must be periodically removed or the quality of the food product will suffer. Interior metal surfaces of the fat-containing vessel and the heat exchanger tubes also become coated with a varnish-like deposit which is a contaminant affecting food quality and also must be periodically removed.
Another cause of shortened fat life comes from water vapor entrained in the food. Surface tension of the fat reduces as it is used for frying. Bubbles of water vapor from the goods being fried then rise through the fat and do not readily burst but remain as bubbles and expand and the fat foams. Foods cooked in foaming fat are excessively greasy, and lack quality.
There is, thus, an established need for a process and apparatus for deep fat frying food products having surface-to-volume ratios which are less than that normally characteristic of potatoe chips and small cut French fries, which process and apparatus would save on fat by displacing non-working fat from the heating and frying portions of the fat fryer and dispensing with the need for ever having to change the fat while maintaining free fatty acid content within acceptable limits by adding make-up fat from time-to-time.